In a positive working photoresist composition, a composition containing an alkali-soluble resin and a naphthoquinonediazide compound, as a photosensitive material, are generally used. For example, "novolac type phenol resin/naphthoquinonediazido-substututed compound" is described in U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,370 as the most typical composition. Examples of "a novolac resin composed of cresol-formaldehyde/a trihydroxybenzophenone-1,2-naphthoquinonediazidosulf onic acid ester" are described in L. F. Thompson, Introduction to Microlithography, No. 219. pages 112-121, published by ACS.
A novolac resin used as a binder is soluble in an alkaline aqueous solution without being swollen with solution, and is particularly useful in the present invention since the novolac resin gives in particular, highly durable plasma etching, in the case where the images formed are used as a mask for etching. Also, when a naphthoquinonediazide compound is used as a photosensitive material, it functions as a dissolution inhibitor by lowering the alkali solubility of the novolac resin by itself. However, it is unique in that when the naphthoquinonediazide compound is decomposed by irradiation of light, an alkali soluble material is formed which acts to increase the alkali solubility of the novolac resin. Owing to the the large property change due to light, the naphthoquinonediazide compound is particularly useful as a photosensitive material for a positive working photoresist.
Hitherto, many positive working photoresists each containing a novolac resin and a naphthoquinonediazide series photosensitive material have been developed and practically used, and they have given sufficient results in line width, working from about 1.5 .mu.m to 2 .mu.m.
However, recently, with the increase in degree of integration of integrated circuits, in the production of a semiconductor substrate, such as a very large scale integrated circuit, etc., an ultrafine pattern composed of a line width of 1 .mu.m or less has been required. For this use, a photoresist having particularly high resolving power and high sensitivity, from the view points of a high pattern form capable of accurately printing the form of a light-exposure mask, and also a high productivity, has been required. The positive working photoreists using conventional naphtoquinonediazide series photosensitive materials, such as gallic acid esters and 1,2-naphthoquinonediazidosulfonic acid esters of polyhydroxybenzophenones as described in U.S. Pat. Nos. 3,046,118, 3,106,465, and 3,148,983 and JP-B-37-18015, JP-B-56-2333, JP-B-62-28457, (the term "JP-B" as used herein means an "examined published Japanese patent applicatin") cannot meet these requirements.
Also, in the formation of an ultrafine pattern composed of a line width of 0.5 .mu.m or less, it has been found, for example, that, even when a certain coated film thickness can provide a predetermind resolving power, even a small change of a coated film thickness can deteriorate the resolving power which can be obtained (this phenomenon is hereinafter referred to as "film thickness dependence"). Unexpectedly, even when the film thickness changes by as small as several hundredth .mu.m, the resolving power changes largely and furthermore it has been found that any one of the typical positive working photoresists which are now commercially available has more or less such a tendency.
Specifically, when the thickness of a resist film before exposure changes from the predetermined film thickness in the range of .lambda./4 n (in which .lambda. is an exposure wavelength and n is the refractive index of a resist film at that wavelength) the resolving power which can be obtained changes accordingly.
The presence of the problem of this film thickness dependence is indicated, for example, in SPIE Proceedings, Vol. 1925, p.626 (1993) describing that this problem is caused by the effect of multiple reflections of light within the resist film.
In the industrial processing of the semiconductor substrates, the patterns are formed using the resist films having the thickness which is uneven very slightly because of unevenness of the substrate surface and of irregular coated film thickness. Therefore, the above noted film thickness dependence is one of the difficult problems in carrying out, using the positive working photoresist, fine processing close to a limitation thereof.
Heretofore there have been proposed numerous photosensitive materials, 1,2-naphthoquinone diazide compounds of polyhydroxy compounds having a specific structure, to improve resolving power, as disclosed, for example, in JP-A-57-63526, JP-A-60-163043, JP-A-62-10645, JP-A-62-10646, JP-A-62-150245, JP-A-63-220139, JP-A-64-76047, JP-A-l-189644, JP-A-2-285351, JP-A-2-296248, JP-A-2-296249, JP-A-3-48249, JP-A-3-48250, JP-A-3-158856, JP-A-3-228057, Toku Hyo-4-502519, JP-A-4-365046, U.S. Pat. Nos. 4,957,846, 4,992,356, 5,151,340 and 5,178,986 and European Patent 530148.
However, even when these photosensitive materials were used, sufficient reduction of the film thickness dependence could not be attained.
On the other hand, that the resists providing high contrast and high resolving power can be obtained by using the photosensitive materials having a hydroxyl group in a molecule, is described, for example, in JP-B-37-18015, JP-A-58-150948, JP-A-2-19846, JP-A-2-103543, JP-A-3-228057, JP-A-5-323597, European Patent 573056, U.S. Pat. Nos. 3,184,310, 3,188,210, 3,130,047, 3,130,048, 3,130,049, 3,102,809, 3,061,430 and 3,180,733, West German Patent 938233 and SPIE Proceedings Vol. 631, p.210, Vol. 1672, p.231 (1992), Vol. 1672, p.262 (1992) and Vol. 1925, p.227 (1993).
It is certain that it is possible to attain high contrast by using the resists containing the photosensitive material described in these publications, but sufficient reduction of film thickness dependence cannot be attained.
As described above, it have not been known at all how the resist materials should be designed in order to reduce film thickness dependence and to obtain high resolving power irrespective of the film thickness.
Also, recently, with the increase in degree of integration of semiconductors, the requirements for the particle of the positive working photoresists have increasingly been enhanced. For semiconductors, as the term " 1/10 rule" has generally been used, the particles having a size of not less than 1/10 of the smallest line width of a device affect the yields (See, for example, Ultraclean Technology, Vol. 3, No. 1, p.79 (1991)).
To reduce the content of these particles, an ultrafine filter having a pore size of 0.1 .mu.m to 0.05 .mu.m is used in the production of the resists, proving to be useful for reduction of the content of these particles in the production of the resists.
However, even if the content of the particles is low in the production of the resists, the content of the particles in the resists frequently increases with time. The increased amount of the particles with time mostly comes from the 1,2-quinonediazide photosensitive materials. Various attempts have been made to reduce the increase of the particles with time.
For example, the use of photosensitive materials in which part of a hydroxyl group of polyhydroxyl compounds are acylated or sulfonylated (JP-A-62-178562), the use of a mixture of a 1,2-naphthoquinonediazido-4-sulfonic acid ester and a 1,2-naphthoquinonediazido-5-sulfonic acid ester (JP-A-62-284354), the use of thermally modified 1,2-naphthoquinonediazide photosensitive materials (JP-A-63-113451), the reduction of the amount of residual catalysts in photosensitive materials (JP-A-63-236030), the synthesis of photosensitive materials in the presence of anion exchange resins (JP-A-63-236031) and the use of photosensitive materials incorporated with solvent easily soluble therein (JP-A-61-260239, JP-A-1-293340) have been attempted.
However, the positive working photoresist compositions using these photosensitive materials have not yet been sufficiently improved for practical use and, also, when these photoresist compositions are stored as solutions, the photosensitive components are liable to form deposits, which frequently causes a problem in their storage stability.